In general, proper lighting for television, motion pictures, theater, and professional photography is difficult to achieve. Lighting systems designed for such use, on one hand must provide very large amounts of light, and on the other hand are limited by the amount of space available, the amount of heat and noise (60 cycle hum) generated by the lights, the amount of available power and overall cost of the system. It is therefore highly desirable to increase the efficiency of any lighting system, commonly measured in terms of lumens per watt, thereby reducing the amount of space, equipment, heat and noise produced by, and power required by a given lighting system.
One of the major factors affecting the operation of a lighting system is the type of lamp employed in any individual lighting unit or "luminaire". One popular lamp developed in the early 1960's is an incandescent lamp in which a gaseous element of the halogen family is provided within the lamp bulb, so as to permit the operation of what is known as the "halogen cycle", which causes particles of the filament which have evaporated therefrom, to be redeposited onto the filament. Such lamps substantially maintain their initial light output over time, without the bulb blackening like a regular incandescent bulb would at high intensities and temperatures. Also, halogen lamps have twice the life of comparable wattage incandescent lamps with the same lumen output, and further, for a given power rating, halogen lamps are more compact than plain incandescent lamps.
The halogen lamps typically employ a quartz bulb, because of the high temperatures that the lamps produce, and have tungsten filaments. Early halogen lamps used iodine as the halogen and were thus referred to as "quartz iodine" lamps, whereas modern-day halogen lamps usually use bromine as the halogen, and are referred to simply as "tungsten-halogen" lamps.
Typical operating characteristics of incandescent lamps, including tungsten-halogen lamps, will be described with reference to FIGS. 1 and 2, which have been duplicated from pages 50 and 51, respectively, of the Lighting Handbook, Seventh Edition, by Sylvania Lighting Center, GTE Products Corp., 1984. FIG. 1 illustrates the variation of light output in lumens, and power consumed, in watts, as a function of the A.C. supply voltage applied to an average incandescent lamp. For example, by operating the lamp at 100% of the rated voltage (e.g., operating a 120 volt A.C. (rms) rated lamp at 120 volts A.C. (rms), the lamp will produce 100% of its rated light output, and will consume 100% of its rated power consumption. FIG. 2 illustrates the variation of the life of an average incandescent lamp as a function of the A.C. supply voltage. For example, operating the lamp at 100% of its rated A.C. voltage will provide an average lamp life equal to the rated life, designated in the figure as 100% of the rated life.
It will be appreciated from FIG. 1 that an increase in the A.C. supply voltage will produce a rapid rise in the lumens produced by the lamp, with a considerably slower rise in the watts consumed by the lamp, thus increasing the light output and efficiency of the lamp. However, it will be appreciated from FIG. 2, that an increase in the A.C. supply voltage produces a rapid drop in the rated life of the lamp. For example, at 10% over the rated A.C. voltage, the life of the average lamp will be reduced to less than 30% of its rated life, while an increase of 20% over the rated A.C. voltage reduces the expected life to an unacceptable value. In fact, the Lighting Handbook, referred to above, states, at page 50 that "Tungsten halogen lamps should never be operated at greater than 10% over voltage."
On the other hand, a decrease in the applied voltage will produce a rapid rise in the rated life of the lamp, as shown in FIG. 2, and such characteristics have led to home use of devices for increasing the life of light bulbs through a reduction in the supply voltage applied to the light bulb, by means of a simple half-wave rectifier which may be easily placed into a light socket. Since the A.C. supply voltage is only applied to the light bulb through one-half of its whole cycle, the rms voltage applied to the light bulb is significantly reduced, resulting in a considerably longer bulb life, lower power consumption, but with a lower light output and a lower lamp efficiency.
Lighting for television, theater and professional photography, however, requires very high levels of light. Simply adding more lamps is not an adequate solution in the industry. Lamps are expensive to rent. Lamps are bulky, and space and outlets on lighting trees are limited. Also, two one-thousand watt lamps do not focus as much light on a subject as one two-thousand watt lamp; the amount of lumens produced may be the same in either case, but the amount of light focused on the subject is not. For shooting at remote locations adding extra lamps is particularly difficult, because of transportation, set up and power generating problems. It is thus desirable to increase the light intensity of individual lamps wherever possible. Converters, such as "Garnelite" converters, made in the early to middle 1960s by J. G. McAlister, Inc., Hollywood, Calif., provided an A.C. to A.C. boost of the voltage applied to tungsten halogen lamps to thereby increase the light output of the lamps. However, the amount of increase in light output provided by the tungsten-halogen lamp using such converters, was severely limited, since a boost of about 10% over rated voltage would dramatically reduce lamp life (FIG. 2), with only a relatively small increase in light output (FIG. 1). Thus, operation at over-voltages resulted in increased expense and down time due to shortened lamp life. Such converters are no longer produced.
In the late 1960s, a new type of lamp known as "HMI" (referring to halogen, mercury and iodine) was developed and first introduced in Europe. The HMI lamp is not an incandescent lamp, but rather is a gas discharge device which inherently operates at higher lumens per watt efficiencies than tungsten-halogen lamps. On the other hand, HMI lamps have serious drawbacks. Each lamp requires a ballast, which can weigh over 250 lbs. (for a 12 kilowatt spotlight, for example). HMI lamps are bulky, expensive, require a great deal of room, require long warm-up and restart periods, are very difficult to dim, and cause flicker problems, which are of particular difficulty in movie photography, due to the pulsed nature of the light. The fact that HMI lamps, given their serious deficiencies, have gained favor in the industry and have been widely used attests to the need for sustainable intensities and efficiencies not attainable with tungsten-halogen lamps, even with boosters of the "Garnelite" type.
Further, both tungsten-halogen and HMI lamps produce sometimes unacceptable levels of noise, referred to as "60-cycle hum". On a shooting set lights have to be raised so that the hum does not interfere with microphones on booms. This, of course, exacerbates the lighting problem, because raising the lights lowers the intensity of light on the subject.